Antenna Coil Problem on AW23

[Joe Miller]

Hi everyone,

I'm up to my ears working on my AW23 (same one I've already got so much help with--much appreciated).

Anyway, I've powered it up and the only way I get any sound is if I put my finger on the grid cap of the RF amp (6D6).  The local station comes in loud and clear when I do that, but nothing when I connect the attenna.

Obviously this sounds like a problem with the antenna coils.  Fortunately, Bruce has shared an AW23 parts chassis with me that I can disassemble to see what's what.

I have found the switch that rotates when the band switch rotates under the can that covers the AM antenna coil. It looks like all the antenna coil secondaries (am coil under one can and the other 3 under another can) have continuity and the switch wiper does connect up to the RF amp grid cap cap.  So the secondary side of the coils seems to be okay.

I have traced the antenna wiring from the antenna post to a coil that surrounds the other 3 antenna coil primaries under their can.  I can't tell how the antenna wiring connects to that coil and I can't get a meter on it.  The schematic indicates that each of the antenna coil primaries are switched via the band switch (I assume like the secondaries are), BUT, I can't find any such switch. 

Anyone know how these antenna coils are configured?  I believe the problem must be somewhere with the connection from the antenna post to the grid of the RF amp?  Any other ideas?

Thanks,

Joe

[Norman S Braithwaite]

Joe:

Scott did not specify polarity of the 9.5-volts.  Set at -9.5 and proceed.  Do not drill new holes in your chassis for installation of the missing link.  You may need to lift the entire back half of the selectivity control mechanism to install the link but it is reasonably possible without corrupting the integrity of your chassis.

Norman

[Joe Miller]

Norman,

Success on the selectivity switch. I was able to get the link installed without drilling any new holes.  Works great now.

The set is playing pretty good now.  I still need to:

• Troubleshoot sensitivity.  I noticed that the sensitivity control does have an affect on the cathode voltage of the noise suppressor (-10v to -20v), but there doesn't appear to be any affect on the sound of a tuned in station when I vary the sensitivity.  I don't think it is working right?
• Figure out why the tuning meter only moves about 1/4 inch--any ideas?  If I am reading it correctly, the AVC voltage goes to the grid of the noise suppressor.  If so, what is the input for the tuning meter amp?  I wonder what voltage on the grid should deflect full scale and what voltage is needed to cut the tube off and put the meter at minimum deflection.  Any ideas?
• Return the bass control to original.  Someone before me has basically gutted the capacitor network attached to the bass control and put one cap in the network's place.  Puzzling, I need to figure this out and put it back to "stock".
• Set up the BFO

I'm a bit apprehensive in doing an alignment because my alignement instructions are hard to read and some of the words are missing.  Do you think an alignment would greatly improve gain and sensitivity?  Maybe this has something to do with the tuning meter?

I thought about replacing the rotating coil assembly with the one from the parts chassis to see what happens, but I probably shouldn't press my luck. 

Joe 

[Norman S Braithwaite]

Joe:

Try the sensitivity control on a weak station, not a strong local.  In the unlikely condition that you do not see a variation in volume on a weak station, isolate the sensitivity control from the bias network and check it.  DO NOT worry about the sensitivity of the tuning meter until after you have a chance to align the receiver and do not swap the coil turret.  A satisfactory alignment can be conducted by setting the selectivity-fidelity to maximum selectivity (full counterclockwise), sensitivity to maximum, and bass control to minimum (not critical), then the following:

1)  Attach the signal generator to the grid of the 6A7 tube

2)  Place a .001 capacitor across the antenna terminals to decouple any stray RF entering through the antenna circuit.

3) measure AC voltage on the high end of the volume control using a VTVM or high impedance VOM.

4) Adjust IF trimmers for peak voltage

This is not a factory recommended alignment but it is essentially similar and will get you very close.  Then you may report on tuning meter response and the IF peak frequency.

Norman

[Joe Miller]

Norman,

I plan to do the IF alignment on Monday.  I can't seem to find the IF frequency on the schematic (465K Hz?)  Do you know what it is?

I assume the IF trimmers are the adjustments I see underneath each of the IF cans, correct?

Which is the best sequence for adjustment 4th IF to 1st or the other way around (or does it even matter)?

Joe

[Norman S Braithwaite]

Joe:

The default IF frequency for the AW-23 was 465-khz.  However, depending on the customers location and frequency of strong locals, EH Scott Radio Laboratories would vary from this frequency in increments of 2.5-khz up to 480-khz.  If the receiver had been tuned to a special IF frequency, the IF frequency would have been marked on the inside of the back apron of the chassis in grease pencil.  For starters, find the peak to which the IF amplifier is presently aligned and peak the amplifier at that point.  Then if you want to move it to 465-khz, move the peak in small increments.  The adjustments are those that are visible under the chassis.  IF alignment requires removal of both the outer and inner shield cans.  When ready to conduct the RF alignment, the inner shield must be replaced first.

Norman

[Joe Miller]

Here's the mod that was done to replace the bad interstage transformer between the first AF and the 2nd AF circuits.  Bass control was also modified a bit.

Seems to work great.

Ready to start the IF alignment.  Will keep you posted on results

Joe

[Joe Miller]

Norman,

IF alignment is complete.  The as found IF frequency was 445.5 KHz.  It was worth the effort.  I increased the output at the top of the volume control from -6.7 vdc to -17.4 vdc.  It plays extremely well on AM.  Will check out my SW sensitivity and quality tonight when SW reception is better.  I will also adjust the BFO tonight.  I have 3 loose ends remaining:

• Sensitivity Control has no affect--still unable to detect any improvement in volume when I adjust it.  The resistor string is okay and the pot isn't open.  I thought I would get quite a bit of swing with this adjustment.  I assume it is supposed to be driving the AVC on the noise supressor tube's grid less negative thereby increasing the gain of the IF amps.  Is that correct?  Any suggestions?
• Tuning Meter Sensitivity--I have more swing on the needle now but it doesn't go full scale (clear across the dial scale).  It does go a bit over half way with a strong station though--maybe that's all the further it is supposed to go?  I did notice the needle is just over half way with the set off.  Is there a spring in there that is supposed to keep it on the right side of the dial with the set off.  Maybe I need a new tuning meter?
• Phono--I've connected the phono input to the high side of the volume control, but can't use the switch that grounds the phono input when it isn't in phono position.  So, I'll remove the antenna leads when I am using the phono, and remove the phono leads when I am using the radio.  BTW, I think this is a ceramic cartridge.  I have it connected as a high input impedance which I think is what the ceramic cartdridge needs.  Is that right?  I think that connection will also work for an IPOD?  When I get everything in the cabinet I will connect the phono and run my finger across the needle  to see if I can hear anything.  If not, I'll get a new cartridge.

Again, thanks for all your help on this.  Once I resolve these loose ends she's going into the cabinet in my family room.  And then on to the next set to fix.

Joe

[Joe Miller]

LIve and learn...I replaced all the resistors in the noise supressor circuit because they had all drifted--that gave me good action at the cathode of the noise supressor tube (76),  But no action on the grid, meaning the sensitivity control still had no affect.  BTW I have been troubleshooting with the set propped up on one end.  Guess what, when I replaced this tube, I must have picked a good one that only works in the vertical position.  When the tube wasn't responding to the good action on the cathode, I checked the tube, it was cold.  Tested in the tube tester and it lit up and tested very good.  So, the filament must have not worked in the horizontal position.  Replaced the tube and now the sensitivity control works great!

I adjusted the BFO last night for a whistle--marginally successful, I got a whistle but not a very loud one.  I'm not sure the BFO feature is worthwhile.

All I have left is the tuning meter.  I'm going to take the dial off and clean it and have a look. Once I figure that out, I'm ready to put it back in the cabinet and play with the  phono.

[Norman S Braithwaite]

Joe:

Man, I'm glad I did not speculate on the sensitivity problem!  I was going to point out that Scott packed the sensitivity control with grease that sometimes hardens over the resistance wire causing the wiper to lift between ends of travel but you had indicated that the control tested OK.  As indicated before the normal range of meter movement is 2/3 of the scale width at best.

Norman

[john stafford]

hi joe can you tell me how hard was it to align the scott 800b i have only did some ajustment on philcos i have 4 scott 800b's but a little chicken to jump in on the scott  i have all of the stuff gen tracer meter just need to get rid of the yellow down my back any help would be great pitfalls and such

john

Joe Miller said:

Norman,

IF alignment is complete.  The as found IF frequency was 445.5 KHz.  It was worth the effort.  I increased the output at the top of the volume control from -6.7 vdc to -17.4 vdc.  It plays extremely well on AM.  Will check out my SW sensitivity and quality tonight when SW reception is better.  I will also adjust the BFO tonight.  I have 3 loose ends remaining:

• Sensitivity Control has no affect--still unable to detect any improvement in volume when I adjust it.  The resistor string is okay and the pot isn't open.  I thought I would get quite a bit of swing with this adjustment.  I assume it is supposed to be driving the AVC on the noise supressor tube's grid less negative thereby increasing the gain of the IF amps.  Is that correct?  Any suggestions?
• Tuning Meter Sensitivity--I have more swing on the needle now but it doesn't go full scale (clear across the dial scale).  It does go a bit over half way with a strong station though--maybe that's all the further it is supposed to go?  I did notice the needle is just over half way with the set off.  Is there a spring in there that is supposed to keep it on the right side of the dial with the set off.  Maybe I need a new tuning meter?
• Phono--I've connected the phono input to the high side of the volume control, but can't use the switch that grounds the phono input when it isn't in phono position.  So, I'll remove the antenna leads when I am using the phono, and remove the phono leads when I am using the radio.  BTW, I think this is a ceramic cartridge.  I have it connected as a high input impedance which I think is what the ceramic cartdridge needs.  Is that right?  I think that connection will also work for an IPOD?  When I get everything in the cabinet I will connect the phono and run my finger across the needle  to see if I can hear anything.  If not, I'll get a new cartridge.

Again, thanks for all your help on this.  Once I resolve these loose ends she's going into the cabinet in my family room.  And then on to the next set to fix.

Joe

[Joe Miller]

Hi John,

I've aligned several Scotts now and had no real problems.  I didn't mess with the SW bands though.  Give it a go.  If you get stuck just post your questions here and there are a number of us who can help you through it.  Keep the input signal magnitude low. Also, I like to keep the AVC bus clamped at a small negative voltage to keep it from influencing the output.  A VTVM or scope is a good way to make sure you can see the peak on the output.  (Digital meters make it harder to make sure you have found the peak.)

Good luck,

Joe

[jimmie o. ashabraner]

   the allwave 23 has a common coil for all antennae input to the wave band switch. start at antennae post and trace to it. the coil is approximately 3/4 inch diameter as I recall. this caused the same trouble on my set.  see if you can find one if it is open ( usually caused by lightning or an electrical spike). mine was burned to a crisp, substituted a small oscillator coil to try it and it worked., did find the right coil later and it works well on shortwave and broadcast bands, good luck, jimmie